Notably, the software update fixes a performance issue with transferring files over 802.11ac. Back in June, it was discovered that OS X didn't scale the TCP window size beyond 64KB when copying files over an AFP share. This artificially limited the speed that users could transfer at.
The prelease build is number "12F35" and includes the following changes.
● Fixes an issue that may prevent Mail from displaying messages
● Improves AFP file transfer performance over 802.11ac Wi-Fi
● Resolves an issue that may prevent a screen saver from starting automatically
● Improves Xsan reliability
You can follow iClarified on Twitter, Facebook, Google+, or RSS to be informed when Apple releases the software update.